Because of the excellent transparency, surface luster, coloring, weather resistance and the like, a transparent resin plate of polymethyl methacrylate, methyl methacrylate, styrene copolymer, polystyrene or the like is used as an ornamental body, such as a signboard, a display and the like, or a roofing member when transparent, or also widely used as a signboard, a billboard, a lightening shade, etc. because of a scattered light permeability when milky in color due to white pigment added to the resin. Especially, a polymethyl methacrylate plate for the former use is significantly excellent in the above-mentioned characteristics and is used most in general.
Many electrical engineers have long aimed great efforts at the development of a surface light emitting body. An electroluminescent (EL) light emitting device utilizing electric fluorescence has already been developed; however, it has not yet been appropriate in practical use because its luminance per unit area is insufficient. Hence, as a surface lightening body, a device in which a light source like a fluorescent lamp is shaded by a light scattering plate of opal glass or cloudy plastic is mainly used.
To still more enhance an ornamental effect of a sheet of the above-mentioned transparent resin, especially a light permeability and a light diffusivity, Japanese Examined Patent Publication No. 16138/1977 discloses a method in which an acrylic resin plate is molded in a foam sheet by forcing the raw material charged with an adequate amount of water to heat up and foam. In accordance with the prior art technique, however, the resultant foam sheet loses a surface smoothness because the foam generated by heating makes concavities and convexities in the surface, and additionally, the inside of the foam is also lacking in smoothness and luster; as a result, the light of incidence on the surface of the sheet immediately attenuates. Thus, the foamed sheet is not suitable for edge lighting; the only thing expected is a scattering effect of the light incident on the sheet. In other words, even if the sheet is used for the purpose of edge lighting, the sheet never shine evenly throughout the surface orthogonal to the incident light by virtue of the foam.
In the above-mentioned surface lighting device, the luminance becomes even as the distance between the light source and the light scattering plate is longer; however, since the luminance is inversely reduced proportional to the distance raised to the second power, the device must have a somewhat complex system design where the distance between the light source and the light scattering plate is long to some extent and the light source consists of a plurality of illuminants appropriately arranged, to harmonize the two contradictory conditions, namely, unifying the luminance and decreasing in brightness.